Process for producing core laminations



United States Patent 3,114,661 PRfiCEdS FUR PRODUCING CORE LAMINATIGNS Bert E. Palm, Cleveland, @hio, assignor to The Luhrizol Corporation, Wicidiife, Ghio, a corporation of Ohio N0 Drawing. Filed Apr. 24-, 1961, der. No. 104,833 4 Claims. (Cl. 148-122) The present invention relates to core laminations used in the manufacture of electrical equipment. In a more particular sense, it relates to core laminations having increased electrical surface resistance.

Core laminations of relatively light gauge (e.g., 0.02 to 0.05 inch) ferrous magnetic stock are employed in the manufacture of electrical equipment such as, for example, transformers, motors, electromagnets, and relays. The use of light gauge low-carbon steel or silicon steel rather than solid bar stock for core laminations reduces electrical losses by lowering the wasteful eddy currents which normally exist in electromagnetic circuits.

It is common practice to provide an insulating material, i.e., core plate, such as asphalt, varnish, paint, metal silicates, or paper between core laminations to insulate them from each other and thereby reduce eddy currents to a minimum. It has also been proposed to treat core laminations with dilute phosphoric old or ferrous acid phosphates containing, optionally, inorganic fillers such as finely-divided silica, montrnorillonite clays, or ground mica to provide an insulating coating. All of these known methods of insulating core laminations are effective in reducing eddy currents but are attended by certain inherent disadvantages.

Organic insulating materials such as asphalt, varnish, or paint are only applied to core laminations which are not to be annealed, because these materials would be destroyed by the high temperatures of an annealing operation. Annealing serves to relieve the strains caused by stamping and to decarburize the ferrous stock and thereby improve its magnetic susceptibility.

In many environments such as oil-cooled transformers, the assembled core laminations are immersed in hydrocarbon oils which dissolve or soften most organic coatings. Furthermore, organic coatings are relatively thick and reduce the space factor (i.e., core density) of the assembled core laminations, resulting in lower electrical efficiency.

The use of known inorganic coatings such as those produced on ferrous magnetic stock by the action of dilute phosphoric acid or ferrous acid phosphates plus, optionally, various inorganic fillers represents an advance over the use of organic coatings. However, known inorganic coatings have a number of disadvantages which seriously restrict their wide application. They invariably possess a massive, hydrated crystalline structure which efiloresces during the annealing operation to yield a discontinuous, poorly adherent coating which tends to dust off or flake from the ferrous surface upon slight abrasion and thus expose portions of the bare metal. These discontinuous coatings are very susceptible to rusting in the presence of humid atmospheres.

Furthermore, when stacks of such coated laminations are annealed, the individual laminations have a tendency to adhere to each other, probably as a result of the fusing of the hydrated crystals between the contacting surfaces. The stacks of core laminations must then be bumped, for example with a mallet, to separate the individual laminations, an operation which is not only inconvenient but which also distorts the laminations and dislodges portions of the coating.

By virtue of the inorganic fillers generally present in known inorganic coatings, the space factor of an assembly of core laminations is appreciably reduced, re-

"ice

sulting in a lowered electrical efiiciency for the assembled core.

A further disadvantage which characterizes known inorganic coatings is their abrasive character, which tends to shorten the life of the dies employed in stamping operations. In some instances this shortening of die life is so serious as to preclude commercial use of a particular coating.

it is an object of the present invention to provide a process for producing core laminations having increased electrical surface resistance and a high space factor.

Another object is to provide a process for producing core laminations having an adherent, micro-crystalline phosphate coating which is resistant to high temperatures, abrasion, and the solvent action of hydrocarbon oils.

A further object is to extend the life of dies employed in stamping core laminations.

A still further object is to provide improved core laminations by a process which is both economical and convenient.

These and other objects will become apparent as the description of the invention proceeds.

in its broadest aspect, the present invention is directed to a process for forming core laminations of increased electrical surface resistance from ferrous magnetic stock which comprises (or) treating said stock with an aqueous phosphating solution to form thereon an integral phosphate coating having substantially no visible crystal structure at a magnification of diameters; and (b) forming core laminations from said treated stock.

In a more particular sense the present invention is directed to a process for producing core laminations of increased electrical surface resistance from ferrous magnetic stock which comprises (a) treating said stock with an aqueous phosphating solution having a total acidity within the range of from about 5 to about 100 points and containing as essential ingredients zinc ion, phosphate ion, an ion selected from the group consisting of nitrate, nitrite, chlorate, perchlorate, and p-erborate ions, and an ion selected from the group consisting of lithium, beryllium, magnesium, calcium, strontium, cadmium, and barium ions to form thereon an integral phosphate coating having substantially no visible crystal structure at a magnification of 100 diameters; and (b) forming core laminations from said treated stock.

in a still more particular sense the present invention is directed to a process for producing core lamination of increased electrical surface resistance from ferrous magnetic stock which comprises (a) treating said stock with an aqueous phosphating solution having a total acidity Within the range of from about 5 to about 100 points and containing as essential ingredients from about 0.1 to about 1.0 percent of zinc ion, from about 0.25 to about 2.0 percent of phosphate ion, from about 0.25 to about 8.0 percent of an ion selected from the group consisting of nitrate, nitrite, chlorate, perchlorate, and perborate ions, and from about 0.1 to about 4.0 percent of an ion selected from the group consisting of lithium, beryllium, magnesium, calcium, strontium, cadmium, and barium ions to form thereon an integral phosphate coating of at least about 25 milligrams per square foot of surface area and having substantially no visible crystal structure at a magnification of 100 diameters; (b) forming core laminations from said treated stock; and (c) annealing said core lminations. If desired, the order of steps (a) and (b) may be reversed when the core laminations are to be annealed; that is, core laminations may be formed from the ferrous magnetic stock, treated with the phosphating solution, and finally annealed.

In its preferred embodiment, the present invention is directed to a process for producing core laminations of increased electrical surface resistance from ferrous magnetic stock which comprises (a) treating said stock with an aqueous phosphating solution having a total acidity within the range of from about 5 to about 50 points and containing as essential ingredients from about 0.1 to about 0.6 percent of zinc ion, from about 0.3 to about 1.5 percent of phosphate ion, from about 0.5 to about 6.0 percent of nitrate ion, and from about 0.1 to about 1.5 percent of calcium ion to form thereon an integral phosphate coating of from about 100 to about 500 milligrams per square foot of surface area and having substantially no visible crystal structure at a magnification of 100 diameters; (b) forming core laminations from said treated stock; and (c) annealing said core laminations at a temperature within the range of from about 900 F. to about 2200 F.

In the ordinary practice of the invention the ferrous magnetic stock, usually either low-carbon steel (0.03 to 0.3 percent carbon) or silicon steel (0.5 to 5 percent Si), is cleaned by physical and/or chemical means to remove any grease, dirt, or oxides and then it is phosphated by means of a solution which forms a micro-crystalline phosphate coating on a metallic surface. if desired, however, the ferrous magnetic stock may be treated prior to phosphating with an aqueous, alkaline permanganate solution so as to condition the surface thereof and make it more receptive to the phosphate coating. Such solution will generally contain from about 0.02 to about 5 percent of an alkali metal permanganate, preferably KMnO and from about 0:03 to about .20 percent of an alkali metal hydroxide, preferably NaOl-I. It may be used at any temperature above the freezing point, preferably from about 100 F. to about 210 F.

The preparation of phosphating solutions or baths adapted for the purpose of the present invention is set forth in co-pending application of John A. Hendricks, Serial No. 373,449, filed August 10, 19 53, and owned by the assignee of the present invention. l1" he phosphating solutions described in the aforesaid application are elfec tive to produce upon metallic surfaces microcrystalline phosphate coatings which show substantially no visible crystal structure at a magnification of 100 diameters. The use of these phosphating baths is an important feature of the present process and it is intended that the disclosure of the said Hendricks application be considered as forming a part of this specification.

In view of the extensive commercial development of the metal cleaning and metal phosphating art and the many journal publications and patents describing the application of phosphating solutions to metal surfaces, it is believed unnecessary to lengthen this specification unduly by a recitation of the many ways in which the phosphating step of the present process may be accomplished. Sufiice it to say that any of the commonly used phosphating techniques such as for example, spraying, brushing, dipping, or rolling may be employed and that the temperature of the aqueous phosphating solution may vary within wide limits, for example, from room temperature to about 212 F. in general however, best results are obtained when the phosphating solution is used at a temperature within the range of from about -1 50 F. to about 210 F. if desired, however, the aqueous phosphating bath may be used at higher temperatures, for example, 225 F, 250 F., or even 300 F. by employing superatmospheric pressures.

The phosphating operation is carried out until the weight of the phosphate coating formed on the ferrous magnetic stock is at least about milligrams per square foot of surface area and is preferably within the range of from about 100 to about 500 milligrams per square foot of surface area. The time required to form the phosphate coating on the ferrous magnetic stock will vary according to the temperature, the type of phosphating solution employed, the particular technique of applying the phosphating solution, and the coating weight desired. Ordinarily the time required to produce a micro-crystal- 4- line phosphate coating of the type preferred for the present invention will be within the range of from about onequarter minute to about 15 or 20 minutes. Under certain circumstances, however, such as the immersion of hot (300-700 F.) steel in a phosphating solution, the steel is phosphated almost instantaneously.

Specific examples of phosphating solutions which yield micro-crystalline phosphate coatings suitable for the purpose of the present invention are shown in the following Table (values given are the percentages by weight of the several ions in the phosphating solution):

TABLE Phosphating Solution Ion A B c D E r o 005 "(iii The points total acid referred to above is an indication of the acidity of a phosphating solution. it represents the number of milliliters of 0.1 normal sodium hydroxide solution required to neutralize a 10 milliliter sample of a phosphating solution in the presence of phenolphthalein as an indicator. Generally a total acidity of from about 5 to about points, preferably from about 5 to about 50 points, is required to obtain phosphating solutions which are capable of providing coma mercially satisfactory coating weights and coating speeds.

The above phosphating solutions can be made conveniently by dissolving zinc dihydrogen phosphate in water to supply the required zinc phosphate ions, and then adding a nitrate, nitrite, chlorate, perchlorate, or perborate of one or more metals of the group consisting of lithium, beryllium, magnesium, calcium, strontium, cadmium, and barium. Finally the acidity of the solution (i.e., the points total acid) is adjusted by the addition of small amounts of phosphoric acid, perch-loric acid, or nitric acid. Alternatively, the solutions can be made by dissolving zinc nitrate, zinc nitrite, zinc chlorate, zinc perchlorate, or zinc perborate in water and then adding a phosphate of at least one metal of the group consisting of lithium, beryllium, rnagnesim, calcium, strontium, cadmium, and barium. As indicated above, the acidity of the resulting solution may then be adjusted by the addition of small amounts of phosphoric acid, perchloric acid, or nitric acid.

The ions of the bath used in the process of this invention may be derived from a variety of compounds and it appears to be of little consequence whether or not these ions come from different salts or acids. Regardless of the identity of the salts selected to provide the required ions, the resulting bath is effective to serve the purpose of this invention. It is necessary only that these salts or acids be used in amounts to provide the necessary concentration of the required characterizing ions. In addition to the characterizing ions present in the phosphating bath, certain supplementary ions such as chloride, bromide, or ammonium ions may also be present to control coating speed, increase the rust-inhibiting qualities of the coating, reduce sludging, etc.

The presence of the lithium, beryllium, magnesium, calcium, strontium, cadmium, or barium ion serves to suppress the formation of massive, hydrated crystalline coatings and yield instead the micro-crystalline or amorphous coating required for the purposes of the present invention. The nitrate, nitrite, chlorate, perchlorate, or perborate ion serves as an oxidizing agent to depolarize the ferrous surface and increase the coating speed of the phosphating solution.

By way of illustration, solution A above was prepared by dissolving in sufficient water to make one liter of solution, 14.2 g. of Zn(NO -6H O, 7.8 g. of commercial 75% H PO 4.2 g. of ZnCl 8.7 g. of NH H PO and Solution B above was prepared by dissolving in sufficient water to make one liter of solution, 21.4 g. of Zn(NO '6I-I O, 7.2 g. of commercial 75% H PO 8.4 g. of NH H PO and 18.1 g. of Ca(NO -3H O.

After the ferrous magnetic stock has been provided with a micro-crystalline phosphate coating, it is rinsed optionally, with water and/ or a hot dilute aqueous solution of chromic acid. This chromic acid solution preferably contains from about 0.01 percent to about 1.0 percent by weight of CrO The hot, aqueous chromic acid rinse appears to seal the phosphate coating and improve its 111st inhibiting properties. For best results, the temperature of the chromic acid rinse solution should be maintained between about 140 F. and 212 F.

After the phosphated ferrous magnetic stock has dried, it is formed into core laminations, generally by means of stamping. These coated, unannealed core laminations may be used as such in the manufacture of electrical equipment. For the reasons previously given, however, it is usually preferred to anneal them.

Core laminations are generally annealed at a temperature within the range of from about 900 F. to about 2200 F., preferably from about 1100 F. to about 1500 F., for a period of at least 0.5 hour and preferably for a period of 2 to 12 hours. The annealing operation may be carried out in an oxidizing atmosphere, an inert atmosphere, or reducing atmosphere. For best results, with core laminations of the present invention, the annealing step is carried out in a mildly reducing atmosphere known in the annealing art as dry cracked gas, a mixture of nitrogen, hydrogen, and carbon monoxide produced by passing a 6.5 1 by volume air: natural gas (methane) mixture over copper catalyst. A comparison of phosphated core laminations which had been annealed in dry cracked gas and air respectively, showed a decided superiority for the former annealing atmosphere. The core lamination which had been annealed in dry cracked gas had a dense, substantially oxide-free coating whereas the coating of the core lamination which had been annealed in air was less dense and covered with a superficial oxide layer (which tended to flake off).

Strongly reducing amospheres such as, pure hydrogen, should be avoided since they tend to reduce the metal phosphates in the coating to the free metal and thus seriously reduce the electrical surface resistance of the coating. To conserve space in annealing furnace, core laminations are generally stacked upon each other. Core laminations prepared in accordance with the process of the pres ent invention show no tendency to adhere to each other after annealing and therefore require no bumping. Furthermore, the coating remains firmly bonded to the metallic surface and shows no tendency to dust off or flake from said surface.

Core laminations prepared in accordance with the process of this invention pass the Franklin Electrical Surface Resistance Test, a test specifically designed to indicate if a core lamination has sufiicient surface resistance to keep eddy current losses at an acceptable level. In this test, two l-centimeter square metal plates are impressed on opposite sides of a core lamination under a pressure of 900 pounds per square inch. An electrical pressure of one volt is applied across the two plates and the amperage flowing through the circuit is measured. An amperage value not greater than 0.5 is indicative of a core lamination which possesses satisfactory electrical surface resistance.

Core laminations of the present invention show amperage values well below 0.5, with the lowest values being shown by laminations which have been annealed.

The following examples are submitted to set forth 6 specific modes of carrying out the process of the present invention. These examples are for the purpose of illustration only and are not to be construed as limiting the scope of the invention.

EXAMPLE I Two strips of low-carbon (0.1 percent C], cold-rolled, 26 gauge steel each measuring 3 inches x 4 feet were thoroughly cleaned by immersing them for 10 minutes a hot (200 F.) aqueous cleaning bath compounded from water and 8 ounces per gallon of a commercial alkaline cleansing powder. The strips were then removed from the cleaning bath, rinsed with tap water, and provided with a micro-crystalline phosphate coating of 290 milligrams per square foot of surface area by immersing them in phosphating solution A for 10 minutes at 200 F.

One phosphated strip (strip W) was rinsed with tap water and the other strip (strip X) was rinsed first with tap water and then with a hot aqueous solution of chrornic acid containing 0.25 gram per liter of CrO After the strips had dried, they were stamped to yield small transformer core laminations. One core lamination from each strip was saved for examination.

The remaining core laminations from each strip were stacked separately. The two stacks were placed in an annealing furnace having an air atmosphere and heated for 4 hours at 1475 F.

The stacks were removed from the furnace and allowed to cool. It was found that the laminations in each stack could be separated by hand without the necessity of bumping. In addition, the coatings on the laminations in each stack showed no evidence of flaking or dusting and were not softened or dissolved by transformer oil.

-An examination of the physical and electrical characteristics of the annealed and the unannealed core laminations provided the following data:

Core lamination processed from strip- Coating magnification or" test value, diameters ampcrcs continuous,

noncrystalhne coating.

How \IUIO EXAMPLE II An experiment similar to Example I was carried out with the exception that the two stacks of core laminations were annealed in an atmosphere of dry cracked gas rather than air.

As in Example I, the annealed core laminations could be separated easily and the coatings showed no evidence of dusting or flaking.

An examination of the physical and electrical characteristics of the annealed and the unannealed core laminations provided the following data:

Core lamination Coating Appearance at :1. Franklin processed from Annealed thickness, magnification of test value strrpmils 100 diameters ampcres W yes 0.15 continuous, 0. 14

noncrystalline coating. no 0.08 0.20 ycs 0. 20 0.08 no 0.05 0.17

of the coating thicknesses of annealed EXAMPLE III An experiment similar to Example I was carried out with the exception that phosphating solution B was used in lieu of solution A.

Here again, the annealed core laminations could be separated with ease and the coatings adhered firmly to the metal surface.

An examination of the physical and electrical characteristics of the annealed and the unannealed core laminations yielded the following data:

Core lamination Coating Appearance at a Franklin processed from Annealed thickness, magnification of test value,

stn'pmils 100 diameters ampercs W yes 0. 2O continuous, 0. 02

noncrystalline X n 0.05 do 0.22

EXAMPLE IV An experiment similar to Example I was carried out with the exception that phosphating solution B was used in lieu of solution A and that an atmosphere of dry cracked gas rather than air was employed in the annealing operation.

After having been annealed, the core laminations from each stack separated easily and the coating remained firmly bonded to the metallic surface.

An examination of the physical and electrical characteristics of the annealed and the unannealed core laminations provided the following data:

Core lamination Coating Appearance at :1 Franklin processed from Annealed thicknes magnification of test value,

stripmils 100 diameters amperes W yes 0.20 continuous, 0. l5

noncrystalline coating. \V 0.08 do 0.18 0. 15 0. 11 0.05 0.22

increased electrical surface resistance from ferrous magnetic stock, which process includes the steps of phosphating said stock with an aqueous phosphating solution and annealing said phosphated stock, the improvement in combination therewith which comprises employing as said aqueous phosphating solution an aqueous solution having a total acidity within the range from about 5 to about 100 points and containing as essential ingredients zinc ion, phosphate ion, nitrate ion, and an ion selected from the group consisting of lithium, beryllium, magnesium, calcium, strontium, cadmium, and barium ions to form on said stock an integral phosphate coating having substantially no visible crystal structure at a magnification of 100 diameters and annealing said phosphated stock in an atmosphere of dry cracked gas.

2. A process in accordance with claim 1 characterized further in that the aqueous phosphating solution employed has a total acidity within the range from about 5 to about 100 points and contains as essential ingredients from about 0.1 to about 1.0 percent of zinc ion, from about 0.25 to about 2.0 percent of phosphate ion, from about 0.25 to about 8.0 percent of nitrate ion, and from about 0.1 to about 4.0 percent of anion selected from the group consisting of lithium, beryllium, magnesium, calcium, strontium, cadmium, and barium ions to form thereon on integral phosphate coating of at least about 25 milligrams per square foot of surface area.

3. A process in accordance with claim 1 characterized further in that the aqueous phosphating solution employed has a total acidity Within the range from about 5 to about points and contains as essential ingredients from about 0.1 to about 0.6 percent of zinc ion, from about 0.3 to about 1.5 percent of phosphate ion, from about 0.5 to about 6.0 percent of nitrate ion, and from about 0.1 to about 1.5 percent of calcium ion to form thereon an integral phosphate coating of from about to about 500 milligrams per square foot of surface area.

4. The process of claim 3 characterized further in that following the step of phosphating but preceding the step of annealing, the phosphated stock is rinsed with a dilute aqueous solution of chromic acid.

References Cited in the file of this patent UNITED STATES PATENTS 2,501,846 Gifiord Mar. 28, 1950 2,743,203 Steinherz Apr. 24, 1956 2,811,473 Allen Oct. 29, 1957 2,975,082 Henricks Mar. 14, 1961 

1. IN THE PROCESS OF PRODUCING CORE LAMINATIONS OF INCREASED ELECTRICAL SURFACE RESISTANCE FROM FERROUS MAGNETIC STOCK, WHICH PROCESS INCLUDES THE STEPS OF PHOSPHATING SAID STOCK WITH AN AQUEOUS PHOSPHATING SOLUTION AND ANNEALING SAID PHOSPHATED STOCK, THE IMPROVEMENT IN COMBINATION THEREWITH WHICH COMPRISES EMPLOYING AS SAID AQUEOUS PHOSPHATING SOLUTION AN AQUEOUS SOLUTION HAVING A TOTAL ACIDITY WITHIN THE RANGE FROM ABOUT 5 TO ABOUT 100 POINTS AND CONTAINIGN AS ESSENTIAL INGREDIENTS ZINC ION, PHOSPHATE ION, NITRATE ION, AND AN ION SELECTED FROM THE GROUP CONSISTING OF LITHIUM, BERYLLIUM, MAGNESIUM,CALCIUM, STRONTIUM, CADMIUM, AND BARIUM IONS TO FORM ON SAID STOCK AN INTEGRAL PHOSPHATE COATING HAVING SUBSTANTIALLY NO VISIBLE CRYSTAL STRUCTURE AT A MAGNIFICATION OF 100 DIAMETERS AND ANNEALING SAID PHOSPHATED STOCK IN AN ATMOSPHERE OF DRY CRACKED GAS. 